1. Field of the Invention
The present invention relates to a reproducing system process of a disk apparatus, and more particularly, to a disk apparatus and a disk reproducing method for decoding a disk modulated in accordance with a modulation rule of a different minimum run length by means of a single Viterbi decoding unit.
2. Description of the Related Art
As a recording medium and a recording and reproducing apparatus capable of recording and reproducing digital data, there can be exemplified an optical disk represented by a DVD (Digital Versatile Disc). For example, in a DVD-RAM which is one of the DVDs, a signal recording layer is provided on a recording medium. A laser light beam having energy which is proper to this signal recording layer is emitted, thereby changing a crystal state of the recording layer. When a laser light beam with proper energy is emitted again to this recording layer, reflection light of an amount according to a crystal state of the recording layer can be obtained. Recording and reproduction of digital data are carried out by detecting this reflection light. As another optical disk, a DVD-RW, a DVD-R or the like is commercially available.
In addition, in recent years, an optical disk apparatus using blue light laser with a short wave-length has also been commercially available. Although these recording media have a plenty of analogies such as identical disk size, they have great differences in the detailed point of view. It is a common object of these recording media to improve a recording density. Further, in order to improve the recording density, a PRML (Partial Response Maximum Likelihood) technique is used.
Now, a principle of the PRML scheme for use in an optical disk apparatus will be described here. A partial response (PR) is provided as a method for carrying out data compression while a necessary signal bandwidth is compressed by actively utilizing an inter-symbol-interference (interference between reproduction signals which correspond to the adjacently recorded bits). Data can be further classified into a plurality of types and classes depending on how to generate inter-symbol interference. For example, in the case of class 1, reproduction data is reproduced as 2-bit data “11” in response to recording data “1”, and inter-symbol interference is generated in response to the succeeding 1 bit. In addition, a Viterbi decoding scheme (ML) is a so called type of a likelihood sequence estimation scheme. This scheme carries out data reproduction based on information on a signal amplitude over a plurality of times by advantageously utilizing a rule on inter-symbol interference possessed by a reproducing waveform. In order to carry out this process, a synchronizing clock synchronized with a reproducing waveform obtained from a recording medium is generated, the reproducing waveform itself is sampled by means of this clock, and the sampled waveform is converted into amplitude information. Then, the amplitude information is converted into a response waveform of a predetermined partial response by carrying out proper waveform equalization. Further, the past and current sample data are used at a Viterbi decoding unit, and the most probable data sequence is outputted as reproduction data. A scheme obtained by combining the above partial response scheme and Viterbi decoding scheme (Maximum Likelihood decoding) is referred to as a PRML scheme.
In the partial response, a reproduction signal sequence can be calculated by making convolution computation of an impulse response of a predetermined partial response class for a recording data sequence. That is, a process from recording to reproduction can be expressed as an arbitrary finite state machine having an N state (in which N=2 m−1 is obtained when a response length of a predetermined partial response is defined as “m”). A two-dimensional graph for expressing (N) of time “k” at which this finite state is present by nodes arranged in a vertical direction, and expressing a transition from each state to each state of time (k+1) as a branch is referred to as a trellis diagram. A Viterbi algorithm is used to obtain a reproduced signal sequence from a reproduction signal sequence, i.e., to make a search for the shortest pass on this trellis diagram. This algorithm is equivalent to a dynamic programming problem to a multi-stepped decision process. A Viterbi decoder based on this algorithm is used to make likelihood estimation of a transmission sequence in a channel having inter-symbol interference and a bandwidth restriction. That is, from among a possible code sequence, for example, a code sequence for minimizing a distance metric (distance function) relating to a sequence of a receive signal such as a sum of a square error in a sequence of the receive signal is selected. In order to use this PRML technique in practice, there is a need for an adaptive equalization technique with high precision and a timing recovery technique with high precision so that a reproduction signal is produced as a response of a predetermined partial response class.
Now, a Run Length Limited (RLL) code for use in the PRML technique will be described here. In a PRML reproducing system, from a signal itself reproduced from a recording medium, a clock synchronized with the reproduced signal is generated. In order to generate a stable clock, it is necessary that the reproduced signal is inverted in polarity within a predetermined time interval. At the same time, in order to reduce a maximum frequency of the reproduced signal, the polarity of the reproduced signal is prevented from being inverted within a predetermined time interval. Here, a maximum data length in which the polarity of the reproduced signal is not inverted is referred to as a maximum run length, and a minimum data length in which the polarity is not inverted is referred to as a minimum run length. A modulation rule in which the maximum run length is 8 bits and the minimum run length is 2 bits is referred to as (1,7)RLL. A modulation rule in which the maximum run length is 8 bits and the minimum run length is 3 bits is referred to as (2,7)RLL. As a typical modulation and demodulation scheme for use in an optical disk, there can be exemplified (1,7)RLL or an EFM plus.
In patent document (Jpn. Pat. Appln. KOKAI Publication No. 2002-344331), an example of a Viterbi decoder circuit is disclosed. With this configuration, for example, reproduction of a DVD-RAM or the like using the (2,10)RLL modulation rule can be carried out.
However, in a prior art of patent document 1, its Run Length Limited is obtained as (2,10)RLL. In the near future, there is a demand for an optical disk apparatus which is compatible with an optical disk medium recorded in accordance with the (1,7)RLL rule. In this apparatus, it is predicted that there is a need for enabling reproduction of a conventional disk medium recorded in accordance with the (2,10) or (2,7)RLL rule. With this configuration, there is a problem that reproduction between a next generation DVD and a current DVD cannot be shared. Furthermore, apart from the (2,10) or (2,7) RLL rule, even if a Viterbi decoder for reproduction of an optical disk medium recorded in the (1,7)RLL rule, there is a problem that remarkable reduction of an area or cost reduction cannot be achieved structurally.